Proposed work continues to be aimed at determining the degree to which the vulnerability of allografts may vary on the basis of differences in the transplant site or the specific tissue grafted. Preliminary results suggest that as examples of endocrine tissue, parathyroids and pancreatic islets differ in that the former is rejected with less and the latter with more vigor than skin transplanted across the same genetic barriers. The behavior of these tissues when allotransplanted to various sites (some immunologically privileged) - peritoneal cavity, anterior chamber, testicle, portal vein, surgically created alymphatic skin islands - will be studied. Extension of function of these endocrine transplants by classical tolerance, enhancement and immunosuppression is also to be investigated. The orthotopic skin allograft has proven to be uniquely exacting in its genetic requirements for success. Prolonged survival of vascularized organ grafts is easier to achieve, possibly because of the mode of grafting or possibly because of a skin specific antigen. Studies will continue to study composite transplants of skin grafts in residence on kidney and then transferred by microvascular anastomosis to allogeneic rats.